The invention relates to a method and apparatus for welding sheet metal blanks along a common edge and following the edge before welding and/or inspecting the edge or weld seam after welding.
The welding together of metal sheets to form larger components (so-called tailored blanks), which are subsequently subjected to a forming process, is known. One example of a field in which such “tailored blanks” are used is the motor vehicle industry. The welding together of the metal blanks may be performed by mesh welding and laser welding. Because the weld seam is subjected to the forming process along with the blanks, it is essential that the entire seam should be of sound quality.
The blanks to be welded are fed to the welding point with the edges to be welded together touching. It is important that the focussed laser beam should follow the common edge of the juxtaposed blanks as accurately as possible. To achieve this, use is made of devices which determine the precise track of the edge ahead of the welding zone, thus enabling the laser beam to be guided accordingly during welding. One known device does this by projecting a line pattern of constant intensity transversely across the edge, this pattern is imaged by a camera, and the image obtained is analysed by computer in order to derive from the trace of at least one of the lines the track of the common edge, or of the gap between the blanks at that point. In this way, the edge and the gap are tracked along their entire length ahead of the welding zone and the laser beam is controlled accordingly as welding is performed.
After the welding zone, the profile of the weld seam can be determined in a similar manner, allowing weld defects to be detected.
Both in following the edge before welding and—especially—in inspecting the edge or weld seam after welding, the problem arises that there is much variation in the reflectivity of the sheet-metal blanks. The individual blanks may have different surface coatings; an oil film, or no oil film, a bright or dull finish, smoke traces, etc. The large differences in brightness due to these factors may go beyond the dynamic range of the camera employed, causing the lines to be imaged too brightly or too dimly and making the image difficult or perhaps even impossible to interpret. If following an edge, it may be necessary to reduce the rate of travel, or weld defects may occur; if inspecting an edge, defects may be difficult to detect in some cases, or defects may be assumed where none exist.
Therefore, the problem which the invention seeks to solve is to improve the method of the kind stated at the outset and/or the apparatus of the kind stated at the outset so as to enable the captured image to be analysed without any problems, even under adverse conditions.